Ethernet is a protocol in the lower layer of the network and operates on the physical (PHY) layer and data link layer in an open system interconnection (OSI) model.
Ethernet is suitable for building a local area network (LAN) because of its simple configuration, flexible networking, low cost, and popularity among people. Developing rapidly, Ethernet is used as a bearer network in metropolitan area networks (MANs) and even in wide area networks (WANs) and Ethernet technology is becoming a mainstream technology in transport networks.
Synchronization is the basis for the digitization of communication networks. Errors and slips in communication networks are inevitable in the absence of normal transmission of synchronous digital information. In a current transport network of Ethernet, information is transmitted asynchronously between nodes. As Ethernet becomes the mainstream technology in transport networks, clock synchronization and transmission need to be realized so that Ethernet can support and become compatible with the time division multiplexing (TDM) service of previous transport networks.
A method for clock transmission and synchronization in an Ethernet in the prior art is Ethernet pseudo wire emulation edge-to-edge (PWE3).
The basic principle of PWE3 is to set up a channel in a packet switched network to implement TDM circuits (T1 or E1) so that it does not matter for the TDM devices at either end of the network whether they are connected to a TDM network. The emulation of a TDM circuit in a packet switched network is known as circuit emulation.
In PWE3, either an external synchronization clock or an algorithm can be adapted to recover a TDM clock, to calculate the original clock rate based on the varying packet arrival rate. As defined in G.823 and G.824 of the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), however, TDM networks have strict requirements on clock stability and the packet arrival rates need to be filtered to lessen the impacts from packet delay variations.
The above method for clock synchronization and transmission is immature and therefore not applied widely because the clock recovery quality in the method is poor in case of heavy traffic.
Another method for clock synchronization and transmission in an Ethernet in the prior art is building integrated timing supply (BITS), in which the network clock is recovered by the global positioning system (GPS).
BITS is the core clock device in synchronization network construction. All devices synchronize their clocks to the precise clock provided by BITS so that precise synchronization of network clocks can be realized. BITS can obtain long-term timing signals of high precision from GPS satellite signals and output timing signals for the reference clock based on a position retaining mode algorithm and a high-quality local oscillator.
The above method is mainly applied in the synchronous digital hierarchy (SDH) of optical networks but not in the Ethernet for clock synchronization and transmission. Special BITS clock network is also needed for the method.